doubleCapital :: [a] -> [a]
doubleCapital [] = []
doubleCapital all@(x:xs) = x:all

bmiTell :: (RealFloat a) => a -> a -> String
bmiTell weight height
    | bmi <= skinny = let s = "You're underweight " in s ++ s
    | bmi <= normal = "You're supposedly normal."
    | bmi <= fat = "You're fat! Lose some weight, fatty!"
    | otherwise = "You're a whale, congratulations!"
    where bmi = weight / height ^ 2
          (skinny, normal, fat) = (18.5, 25.0, 30.0)